In the field of epifluorescence microscopy biochemical material samples that have been Lagged with fluorescent markers often are imaged with a beam of light that excites markers within the samples to fluoresce. The fluorescence light emitted from the sample depends on the markers that have been used with the sample. While a sample is illuminated with excitation light, a microscope images fluorescence light emitted from the sample onto an image detector, such as a CCD array or image tube, in a camera, to determine the spatial distribution of fluorescence light intensity.
It is apparent that the emitted fluorescence light must be distinguished from the excitation light to the detect spatial distribution of fluorescent markers in the sample. This requires spatially separating from a common optical pathway at the sample the respective optical pathways of the incoming excitation light and the outgoing fluorescence light. This is ordinarily accomplished using a wavelength-selective mirror, that is, a device that accepts an excitation light beam emitted by an excitation light source along a source pathway and directs it along the common pathway, and receives fluorescence light emitted by an excited sample along the common pathway and directs it along a detector pathway to a camera. While other types of wavelength-selective devices might be used, dichroic mirrors are particularly suitable for this application because, in addition to their optical property of reflecting light having one wavelength and passing light of different wavelengths, they are typically thin and lightweight.
Often it is desirable to use several different wavelength-selective mirrors in an epifluorescence microscope to accommodate different excitation and emission wavelengths. This requires some mechanism to switch from one mirror to another.
One way of switching mirrors is to use a sliding elongate carriage that has two or more alternative mirrors mounted thereon adjacent one another so that, as the carriage is moved back and forth along its elongate axis, one of the mirrors is selected to interrupt the common pathway of the excitation and emission light beams. However, such mechanisms have relatively slow switching speeds because of the inertia that must be overcome to replace one mirror with another. This limits the speed with which epifluorescence microscopy can be used to process samples. The starting and stopping of such linear devices also tends to impart vibrational energy to the entire imaging instrument.
Another known way of switching mirrors is to mount them on a rotating carousel whose axis of rotation is parallel to the optical axis of the light beam whose wavelengths require separation. This reduces the vibration problem, but makes less certain the registration of different images produced by different fluorescence wavelengths. It also suffers from the problem of inertia where the mirrors are required to start and stop, thereby limiting the speed with which measurements can be made.
Consequently, there is a need for a faster mechanism for switching wavelength-selective mirrors, particularly in wide-field, imaging epifluorescence microscopy where the registration of images at different wavelengths is important.